ProSoundWeb Community

Sound Reinforcement - Forums for Live Sound Professionals - Your Displayed Name Must Be Your Real Full Name To Post In The Live Sound Forums => LAB Lounge => Topic started by: Kevin Maxwell on January 22, 2016, 11:48:35 am

Title: Y-ing a signal into a mixer with Phantom power already on it.
Post by: Kevin Maxwell on January 22, 2016, 11:48:35 am
Y-ing a signal into a mixer with Phantom power already on it.

This is for a SMAART rig. I have just changed the way I inject pink noise into the mixer. I now have a noise plug (pink noise in an XLR) it needs Phantom power. I also need Phantom power for the measurement mics, the USB preamp audio interface has global Phantom power. I want to use an XLR Y cord to route the pink noise into one channel of the USB preamp and one channel of the console. This means that I will be putting Phantom power into the mixer from the USB preamp. Is this a no no? I was also thinking of making a Y cable with a female XLR to one male XLR and one TRS ¼” connector for when I want to go into a ¼” input instead of an XLR input. This also brings up the question of injecting 48v into the mixer into the ¼” input. I just used this for the first time the other day and I had a Whirlwind Line Balancer/Splitter handy. So first I tested to see if it was linear and it was, so I used that between the noise plug and the mixer just to be safe.     
Title: Re: Y-ing a signal into a mixer with Phantom power already on it.
Post by: Mac Kerr on January 22, 2016, 12:01:40 pm
Y-ing a signal into a mixer with Phantom power already on it.

This is for a SMAART rig. I have just changed the way I inject pink noise into the mixer. I now have a noise plug (pink noise in an XLR) it needs Phantom power. I also need Phantom power for the measurement mics, the USB preamp audio interface has global Phantom power. I want to use an XLR Y cord to route the pink noise into one channel of the USB preamp and one channel of the console. This means that I will be putting Phantom power into the mixer from the USB preamp. Is this a no no? I was also thinking of making a Y cable with a female XLR to one male XLR and one TRS ¼” connector for when I want to go into a ¼” input instead of an XLR input. This also brings up the question of injecting 48v into the mixer into the ¼” input. I just used this for the first time the other day and I had a Whirlwind Line Balancer/Splitter handy. So first I tested to see if it was linear and it was, so I used that between the noise plug and the mixer just to be safe.   

Why not just use the noise generator built into Smaart? You don't need the stick, just take one out of your interface to the console, and one can either loop back into an input for the reference input, or that can be done internally.

Mac
Title: Re: Y-ing a signal into a mixer with Phantom power already on it.
Post by: Kevin Maxwell on January 22, 2016, 02:19:01 pm
Why not just use the noise generator built into Smaart? You don't need the stick, just take one out of your interface to the console, and one can either loop back into an input for the reference input, or that can be done internally.

Mac

Sometimes I am working on someone’s system that the mixer doesn’t have a built in Pink noise generator. Or I don’t have the time to try to figure out how to make it work the way I want. So I have standardized on supplying the pink noise from outside the console.
Title: Re: Y-ing a signal into a mixer with Phantom power already on it.
Post by: Tim McCulloch on January 22, 2016, 02:46:28 pm
Sometimes I am working on someone’s system that the mixer doesn’t have a built in Pink noise generator. Or I don’t have the time to try to figure out how to make it work the way I want. So I have standardized on supplying the pink noise from outside the console.

Mac is referring to the noise generator built into SMAART, not the console.  You might want to re-read his reply.

That said, I put a 10 minute WAV file of pink noise (edited from Bink's Audio Test CD) on my audio player for when I just need some pink noise and not using Smaart.
Title: Re: Y-ing a signal into a mixer with Phantom power already on it.
Post by: Kevin Maxwell on January 22, 2016, 03:52:27 pm
Mac is referring to the noise generator built into SMAART, not the console.  You might want to re-read his reply.

That said, I put a 10 minute WAV file of pink noise (edited from Bink's Audio Test CD) on my audio player for when I just need some pink noise and not using Smaart.

WOW my reading comprehension is sorely lacking today. I keep going to bed late and waking up early because of a few things in the works on my mind. So I am a bit sleep depraved and yes I did mean to say depraved and not deprived. 

I don’t know if it is still the case but there was a problem of getting accurate times with the internal pink noise and the impulse measurements and I use that capabilities a lot to set delay speaker times. So I never got into using the internal generator.
Title: Re: Y-ing a signal into a mixer with Phantom power already on it.
Post by: Mac Kerr on January 22, 2016, 04:35:08 pm
WOW my reading comprehension is sorely lacking today. I keep going to bed late and waking up early because of a few things in the works on my mind. So I am a bit sleep depraved and yes I did mean to say depraved and not deprived. 

I don’t know if it is still the case but there was a problem of getting accurate times with the internal pink noise and the impulse measurements and I use that capabilities a lot to set delay speaker times. So I never got into using the internal generator.

There was an issue with the absolute time displayed, but since you are always only interested in the time difference between two signals the offset error cancels out. This was only when using the internal signal path. It was never an issue coming out of your interface and back in for reference.

Mac
Title: Re: Y-ing a signal into a mixer with Phantom power already on it.
Post by: Kevin Maxwell on January 22, 2016, 05:57:48 pm
There was an issue with the absolute time displayed, but since you are always only interested in the time difference between two signals the offset error cancels out. This was only when using the internal signal path. It was never an issue coming out of your interface and back in for reference.

Mac

Thank you for the help on that way of doing it I will have to play with it again, it didn’t work properly that last time I tried it but I had a different USP preamp back then.

But can anyone answer my original question about Phantom power any Y-ing?
Title: Re: Y-ing a signal into a mixer with Phantom power already on it.
Post by: David Sturzenbecher on January 22, 2016, 11:25:55 pm
Thank you for the help on that way of doing it I will have to play with it again, it didn’t work properly that last time I tried it but I had a different USP preamp back then.

But can anyone answer my original question about Phantom power any Y-ing?

This is the exact same setup as a monitor console applying phantom power to a passive split, and the FOH console is seeing the 48V.   It happens 1000's of times a day, and is perfectly fine. XLR inputs are all generally fine with having phantom applied by an external source, but the same cannot be said about 1/4" inputs.
Title: Re: Y-ing a signal into a mixer with Phantom power already on it.
Post by: Kevin Maxwell on January 22, 2016, 11:41:03 pm
This is the exact same setup as a monitor console applying phantom power to a passive split, and the FOH console is seeing the 48V.   It happens 1000's of times a day, and is perfectly fine. XLR inputs are all generally fine with having phantom applied by an external source, but the same cannot be said about 1/4" inputs.

I always had transformer isolated splits, even 3 ways when there was also a broadcast split. One went direct and the other two were transformer isolated. And even that didn’t help when you had Vari-Lites all over the stage and the cabling was almost intertwined. Sorry flashing back to shows past. 

I believe you and it makes a lot of sense. I was thinking of the times when I had to feed a console out to something that had phantom on it. I got in the habit of transformer isolating everything. I just didn’t want to mess up someone else’s console.

Thank you for the reply. And I won’t make the Y cable to ¼” version.
Title: Re: Y-ing a signal into a mixer with Phantom power already on it.
Post by: Jordan Wolf on January 23, 2016, 01:57:12 am
Transformer isolation is your friend.


- Jordan Wolf
Title: Re: Y-ing a signal into a mixer with Phantom power already on it.
Post by: Steve M Smith on January 23, 2016, 04:16:29 am
This is the exact same setup as a monitor console applying phantom power to a passive split, and the FOH console is seeing the 48V.   It happens 1000's of times a day, and is perfectly fine.

Yes - If an input can stand having 48v phantom applied internally, it can stand it when applied externally from another mixer.

There are a lot of phantom power myths out there.  I saw a hilarious one a couple of days ago.  It was a forum post about removing the transformer from an SM57.  Someone warned against the 'danger' of running it with phantom power on "unless you know for sure that your 6K8 resistors are 1% precision types so both pins see the same voltage".

1.  No current can possibly flow from +48v to +48v so no damage can occur.
2.  Even if current did flow, the resistors limit it to a level which could not burn out the winding of a dynamic mic.
3.  Without a current flow there XLR inputs are all generally fine with having phantom applied by an external source, but the same cannot be said about 1/4" inputs. is no voltage drop across the resistors so their precision is irrelevant,

This was almost as funny as the phantom power with ribbon microphones mythology.


Steve.
Title: Re: Y-ing a signal into a mixer with Phantom power already on it.
Post by: James Rowe on January 23, 2016, 05:51:48 am
I blew up a mixwiz a few years ago plugging it's main out into a phantom powered XLR on another mixer. Up until then I would have agreed with you!
Title: Re: Y-ing a signal into a mixer with Phantom power already on it.
Post by: Steve M Smith on January 23, 2016, 06:40:14 am
I blew up a mixwiz a few years ago plugging it's main out into a phantom powered XLR on another mixer. Up until then I would have agreed with you!


I didn't say anything about outputs, I was referring to parallel inputs - which is standard practice.


However, I would be very surprised if a MixWizard output couldn't stand having phantom power applied.


Steve.
Title: Re: Y-ing a signal into a mixer with Phantom power already on it.
Post by: James Rowe on January 23, 2016, 06:53:21 am

I didn't say anything about outputs, I was referring to parallel inputs - which is standard practice.


However, I would be very surprised if a MixWizard output couldn't stand having phantom power applied.


Steve.

Tell that to my repair guy. The mixer still lit up but no sound. A couple of burnt out resistors inside.
Title: Re: Y-ing a signal into a mixer with Phantom power already on it.
Post by: gordonmcgregor on January 24, 2016, 06:40:37 am
Tell that to my repair guy. The mixer still lit up but no sound. A couple of burnt out resistors inside.
Steve you'd be suprised how easily Phantom can kill an output on modern electronically balanced kit these days, ask anyone who has had the misfortune to have a vidiot leave phantom switched on on his camera when you send him a feed
Title: Re: Y-ing a signal into a mixer with Phantom power already on it.
Post by: Keith Broughton on January 24, 2016, 06:53:21 am
Not knowing what USB interface you are using, this might not be applicable.
My Behringer 404 has global phantom but the XLR inputs are combo type with TRS as well.
I feed the return noise to the TRS inputs as they don't have phantom power on them.
As for the transformer iso, correct me if I am wrong but would a transformer (on just the one side of the Y)not create phase shift to the reference input?
Title: Re: Y-ing a signal into a mixer with Phantom power already on it.
Post by: Rob Spence on February 13, 2016, 12:03:55 pm
Steve you'd be suprised how easily Phantom can kill an output on modern electronically balanced kit these days, ask anyone who has had the misfortune to have a vidiot leave phantom switched on on his camera when you send him a feed

I use iso any time I feed someone else.


Sent from my iPad using Tapatalk HD
Title: Re: Y-ing a signal into a mixer with Phantom power already on it.
Post by: Scott Holtzman on February 13, 2016, 10:15:54 pm
Not knowing what USB interface you are using, this might not be applicable.
My Behringer 404 has global phantom but the XLR inputs are combo type with TRS as well.
I feed the return noise to the TRS inputs as they don't have phantom power on them.
As for the transformer iso, correct me if I am wrong but would a transformer (on just the one side of the Y)not create phase shift to the reference input?

A capacitor in series will block DC.